Mutable alleles at the R locus.
Further study of the unstable alleles at the R locus, in chromosome X, has permitted several conclusions regarding the genetic behavior and nature of these mutants. As indicated in the previous report (Maize Genetics Coop. News Letter 23: 1949). these mutable loci are of independent, spontaneous origin, and all derive from the same parental rch gene.
Phenotypic observations, in conjunction with genetic transmission tests, further strengthen the proposed hypothesis that the timing and frequency of reverse mutation is under genic control. Moreover, it is believed that the control is a function of the altered state of the locus itself.
The genetic behavior of the unstable alleles is perhaps best demonstrated by a series of single anther pollinations from individual sectorial plants. Particularly instructive are those progenies from plants representing infrequent but early mutational changes. The type cross employed was Rr / rch‑mut x rg/ rg.
Progeny tests of the pollen taken from fully purple anthers within the sector yield, for the most part, in the colorless seed category, plants which are effectively identical to the unmutated parental rch allele in phenotype. This suggests, clearly, that the mutational change occurred sufficiently early in development as to affect a majority, and in many cases, all of the gametes within the sampled area. Since in this, and in all other instances, sibs derived from the colored seed failed to exhibit an altered phenotype, it is concluded that the mutational changes specifically involve only one member of the allelic pair; namely, the newly established unstable rch loaus.
In the case of outcross tests of single green anthers from the same tassel the resultant progeny from colorless seed is largely composed of green plants, phenotypically equivalent to rg, the bottom member of the series. However, in such crosses an occasional plant may be found which shows a few mutations very late in development. Such late mutations are observable only in the anther wall. Single anther tests from such areas give progeny which is about the same as that derived from wholly green anthers.
Testing a gametic sample from an area representing infrequent, but comparatively early mutation gave somewhat unexpected results. In the progeny of this test may be found a variety of sectorials, differing in both time and frequency of mutation, in addition to a raage of stablized intermediates, full rch reversions, and plants indistinguishable from rg.
The recovered intemediate and full rch alleles are quite stable, but in several cultures an occasional sectorial plant has been observed. While in these particular cases it was not possible to rule out pollen contamination,, other evidence makes this seem improbable. A number of ears, carrying Pl, derived from recovered full rch reversions showed sectors for late loss of pericarp color. Since these sectorial pericarp‑seeds have given rise to variegated plants, the findings would seem to indicate that the full rch alleles recovered from reverse mutation of the mutable locus are not nearly as stable as the original parental gene.